Radiotherapy is used to treat tumors in mammalian tissue, such as the brain. One such radiotherapy technique is Stereotactic Radiosurgery (SRS), by which a high dose of radiation is delivered with high precision over a short course of treatment to a lesion localized relative to a three-dimensional reference system. During SRS, multiple radiation beams are directed towards a target, such as a tumor, from different angles. These radiation beams converge at an isocenter. Tissue located at the isocenter receives a high dose while the surrounding areas receive relatively lower doses. Therefore, it is critical for an SRS system to deliver doses to a volume that conforms to the shape of the target. In this way, the target receives the required dose and the dose received by the surrounding tissue, often called the organ at risk (OAR), can be minimized.
In certain cases, the target may be surrounded by OARs that may be sensitive to radiation. As a result, the doses received by these OARs have to be limited to a predetermined level. Such limitations on the doses received by the OARs, often called constraints, need to be satisfied during treatment planning.
The constraints may be satisfied by manipulating the placement of the beam source. In an SRS system the beam source is often rotated along an arc, where the placement of the beam source is referred to as arc placement. In those instances where a tumor is close to critical OARs or where a tumor has a noncircular shape, manipulation of arc placement alone may not be able to satisfy the constraints.
One way of treating a noncircular or irregular shaped tumor is to combine multiple beam focus areas, also referred to as isocenters, into a noncircular or irregular shape that conforms to the shape of the tumor. FIGS. 1A-1C illustrate the concept of this method. As shown in FIGS. 1A and 1B, an isocenter (e.g., 10 or 20) associated with a group of arcs (e.g., arc group [12A, 12B] or [22A, 22B]) may have a circular (e.g., in a 2D image) or spherical (e.g., in a 3D image) shape. To map the elongated oval shape of a target 32 shown in FIG. 1C, isocenters 10 and 20 may be combined (e.g., by combining their associated groups of arcs) to form a combined isocenter (e.g., 30) that conforms to the shape of the target.